1. Technical Field
Example embodiments relate to a method for resetting and driving a time-based CMOS image sensor, for example, to a method for resetting and driving a time-based CMOS image sensor which may be operated at a low illumination by detecting a fine amount of light, where an intensity of the light may be low, by allowing a reset electron potential of a photodiode to be higher than that of a floating diffusion node by at least an offset of a comparator during resetting of the photodiode and the floating diffusion node.
2. Description of Related Art
In general, image sensors capture images using a characteristic of a semiconductor that responds to light. Objects existing in nature may have different brightness and wavelengths with respect to light and thus show a different electrical value at each pixel of a detector. The image sensor may convert the electrical value to a level that may be signal processed. Image sensors may be widely used, for example, in a video conference camera, a digital still camera, a PC camera, and more generally, a personal portable communication apparatus having a function to transfer image information.
The image sensor includes a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor. The CMOS image sensor may be increasingly used compared to the CCD image sensor because a driving method is simple, embodiments in a variety of scanning methods are possible, a single processing circuit may be integrated in a single chip so that a compact product may be manufactured, a CMOS manufacturing technology is used so that a manufacturing cost may be reduced, and power consumption is low.
However, in the CMOS image sensor, a power voltage may not be lower than a particular voltage due to a limit in light signal input of a photodiode. Also, although a pixel size is generally decreased as desired image quality increases, the pixel size cannot be decreased below a particular size due to a fill factor of the pixel.
To address these problems, a time-based CMOS image sensor capable of operating at a low voltage power has been developed. The time-based CMOS image sensor may measure the amplitude of a signal in terms of time by applying a ramp signal in a pixel.
FIG. 1A is a circuit diagram of a conventional time-based CMOS image sensor. FIG. 1B is an electron potential diagram during resetting and operation of the conventional time-based CMOS image sensor.
Referring to FIG. 1A, a unit pixel 10 of the conventional time-based CMOS image sensor includes a reset transistor Rx, a transfer transistor Tx, a photodiode PD, a floating diffusion node FD, a source follower transistor Dx, and a pixel selection transistor Sx. Although not illustrated, a comparator may be electrically connected to a source of the pixel selection transistor Sx.
In FIG. 1A, in a general time-based CMOS image sensor, the photodiode PD and the floating diffusion node FD are reset to the same reset voltage VDD−VT by the reset transistor Rx. Where, VDD is a power voltage and VT is a threshold voltage of the transfer transistor TX or the reset transistor RX. Thus, referring to FIG. 1B, the reset electron potential of the floating diffusion node FD is the same as that of the photodiode PD. Even when a ramp signal is applied to the transfer transistor Tx after exposure, the time-based CMOS image sensor is not operable if the reset electron potential or voltage of the photodiode PD is lower than the reset electron potential of the floating diffusion node FD raised by a comparator offset. The comparator offset signifies the minimum voltage that may be detected by the comparator. Thus, the conventional time-based CMOS image sensor is not operable at a low illumination because the difference in the reset electron potential from the photodiode PD to the floating diffusion node FD is not large enough to overcome the comparator offset. As a result, a digital correlated double sampling (CDS) method may not be applied to the time-based CMOS image sensor so that the time-based CMOS image sensor may be very sensitive to noise.